Great St. Martin Church has dominated Cologne's old town skyline for 800 years, its distinctive tower with four corner turrets rising above the Rhine waterfront. The 12th-century church was built on the foundations of Roman warehouses that once stood on an island between two branches of the Rhine. Those foundations are still there, visible beneath the church for a small entrance fee.

The church was a Benedictine monastery for most of its life. Its interior furnishings and ceiling paintings were lost in the Second World War, and what stands today is largely a post-war reconstruction. The exterior survives in its Romanesque form: solid stone, small round-arched windows, and a symmetry that impressed even in the Middle Ages.

VoiceMap's self-guided audio tours use the church to trace the layers beneath Cologne, connecting the Roman warehouse island to the medieval monastery and explaining how both shaped the city's harbour and old town.